1. Field of the Invention
The invention relates to fabrication of optical fiber, in particular preparation of the preform from which fiber is drawn.
2. Discussion of the Related Art
Optical fiber is produced from a glass preform. The preform is generally arranged vertically in a draw tower such that a portion of the preform is lowered into a furnace region. The portion of the preform placed into the furnace region begins to soften, and the lower end of the preform forms what is known as the neck-down region, where glass flows from the original cross-sectional area of the preform to the desired cross-sectional area of the fiber. From the lower tip of this neck-down region, the optical fiber is drawn.
Optical transmission fiber typically contains a high-purity silica glass core optionally doped with a refractive index-raising element such as germanium, an inner cladding of high-purity silica glass optionally doped with a refractive index-lowering element such as fluorine, and an outer cladding of undoped silica glass. In some manufacturing processes, the preforms for making such fiber are fabricated by forming an overcladding tube for the outer cladding, and separately forming a rod containing the core material and inner cladding material. The core/inner cladding are fabricated by any of a variety of vapor deposition methods known to those skilled in the art, including vapor axial deposition (VAD), outside vapor deposition (OVD), and modified chemical vapor deposition (MCVD). MCVD is discussed in U.S. Pat. Nos. 4,217,027; 4,262,035; and 4,909,816. MCVD involves passing a high-purity gas, e.g., a mixture of gases containing silicon and germanium, through the interior of a silica tube (known as the substrate tube) while heating the outside of the tube with a traversing oxy-hydrogen torch. In the heated area of the tube, a gas phase reaction occurs that deposits particles on the tube wall. This deposit, which forms ahead of the torch, is sintered as the torch passes over it. The process is repeated in successive passes until the requisite quantity of silica and/or germanium-doped silica is deposited. Once deposition is complete, the body is heated to collapse the substrate tube and obtain a consolidated core rod in which the substrate tube constitutes the outer portion of the inner cladding material. To obtain a finished preform, the overcladding tube is typically placed over the core rod, and the components are heated and collapsed into a solid, consolidated preform. It is possible to sinter a porous overcladding tube while collapsing it onto a core rod, as described in U.S. Pat. No. 4,775,401.
Because the outer cladding of a fiber is distant from transmitted light, the overcladding glass does not have to meet the optical performance specifications to which the core and the inner cladding must conform. For this reason, efforts to both ease and speed manufacture of fiber preforms focused on methods of making overcladding tubes. One area of such efforts is the use of a sol-gel casting process.
U.S. Pat. No. 5,240,488 (the ""488 patent), the disclosure of which is hereby incorporated by reference, discloses a sol-gel casting process capable of producing crack-free overcladding preform tubes of a kilogram or larger. In the process of the ""488 patent, a colloidal silicon dispersion, e.g., fumed silica, is obtained. To maintain adequate stability of the dispersion and prevent agglomeration, the pH is raised to a value of about 11 to about 14 by use of a base, and the dispersion is then aged. Subsequent to aging, as discussed in Col. 15, lines 39-65 of the ""488 patent, a gelling agent such as methyl formate is added to the dispersion to lower the pH. Typically, once the gelling agent is added, but before gellation occurs, the mixture is pumped into a tubular mold containing a central mandrel, and the gel is aged in the mold for 1 to 24 hours. The mandrel is removed, and the gelled body is then extracted from the mold. The body is then dried, fired to remove volatile organic materials and water,. and then sintered to form the finished overcladding tube. The tube can then be used to form conventional preforms.
There are several difficulties typically encountered in forming preforms. These include insertion of the rod into the overcladding tube the small clearances demand that the rod and tube be extremely straight, which is difficult in practice. The amount of heat required to collapse a thick-walled overcladding onto a tube is considerable, and often requires a specialized furnace or plasma torch. In addition, some overcladding tubes are treated with an additional plasma etch to smooth their interior prior to placing the core rod therein, and such additional process steps are advantageously avoided. Also, collapsing a tube onto a rod tends to create inhomogeneous sites or nucleation centers for bubbles, and these bubbles can result in undesired airlines in the drawn fiber.
Thus, improved techniques for assembling preforms and drawing fiber from preforms, particularly with sol-gel overcladding tubes, are desired.
The invention provides an improved technique for assembling and drawing fiber from preforms. In one embodiment, the technique involves providing a core rod assembly that comprises a core rod, optionally with an overcladding layer formed thereon. (Overcladding layer indicates that the overcladding material is located directly on the surface of the rod.) The core rod assembly comprises a handle at a first end and a centering bushing attached at a second end of the assembly. The core rod assembly is inserted into an unsintered overcladding tube such that there is an annular gap between the assembly exterior and the tube interior. The first end of the assembly is typically secured to the tube, by use of the handle, such that the core rod assembly is suspended within the tube, and the core rod assembly and tube are typically suspended by the handle for further processing. The overcladding tube and the core rod assembly are then heated to sinter the overcladding tube and thereby form a preform assembly.
During the heating step, the centering bushing comes into contact with the interior of the overcladding tube, and, because the bushing has a larger diameter than the core rod (or core rod plus overcladding layer), most of the annular gap between the core rod assembly and the overcladding tube is maintained. It is then possible to attach a draw handle to the preform assembly, place the preform assembly into a draw tower, and draw fiber from the preform assembly. Specifically, in the draw furnace, the annular gap is evacuated through the draw handle, and the end of the preform opposite the draw handle is lowered into the draw furnace. The combination of the high temperatures, e.g., 2000 to 2200xc2x0 C. and the reduced pressure in the annular gap induce the tube to collapse onto the core rod as the whole assembly is fed into the furnace. It is possible to perform the process such that the preform assembly contains two or more coaxial overcladding tubes around the core rod, e.g., where the core rod assembly comprises one or more sintered or unsintered overcladding tubes.
Drawing fiber from a preform assembly containing a core rod secured within an overcladding tube, with an annular gap between the rod and tube, has been previously used, but only by insertion of a core rod into a sintered tube. See, e.g., co-assigned patent application Ser. No. 09/515,227, entitled xe2x80x9cApparatus and Method for Making Multiple Overclad Optical Fiber Preforms and Optical Fiber Therefrom,xe2x80x9d filed Feb. 28, 2000. The invention, however, provides numerous advantages where overcladding tubes are produced by sol-gel techniques, or other techniques that result in production of an unsintered tube. For example, the typical process sequence for a gel tube is sinter, plasma etch the tube bore, insert a core rod, collapse the tube onto the rod, and draw fiber from the monolithic preform. According to the invention, the plasma etch is no longer necessary since the tube is not collapsed onto the rod, and separate sintering and collapse steps are avoided. Moreover, because an unsintered tube has a larger inner diameter than the final, sintered tube, the clearance for inserting a rod into the tube is greater, and this clearance eases the insertion, reduces the number of damaged tubes and rods, and relaxes the specifications for bow of the tubes and rods.